marked as duplicate by Michael C canonUsers with the canon badge can single-handedly close canon questions as duplicates and reopen them as needed.Jun 17 '18 at 16:55

1 Answer
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This is probably because some cameras disable autofocus at slow apertures. The camera manufacturers think that performance won't be good enough, and so they disable it. The third-party lens is reporting inaccurate information to the camera so that it will work anyway — slow and sometimes incorrect, perhaps, but at least the camera will try.

Exposure might be off by a tiny bit as a result, but the third-stop difference between f/5.6 and f/6.3 basically comes out in the wash anyway.